COGGING TORQUE

Cogging torque is the oscillating torque determined by the tendency of the rotor to align with the stator in a direction where the permeance of the magnetic circuit is raised to the maximum as seen by the magnets and this will exist even without current in the stator. When the electric motor is rotating 10hp motors, oscillating torque components can arise from the interaction of the magnets with the harmonics due to the winding layout and the current harmonics of the converter.

These oscillating torque components are electromagnetic and are called a torque ripple, while the term cogging torque is reserved for the zero current condition. In a well designed engine, torque ripple and cogging torque can be neglected. A large number of grooves / pole decreases the cogging torque, due to the relative permeability variation of the magnets getting reduced as it successively passes through the groove and the stator tooth. A small incline either in the stator or in the rotor will be sufficient to considerably decrease the cogging torque. When the number of grooves / pole is close to one, the groove geometry becomes more important, and the width of the stator teeth can be adjusted to minimize the cogging torque effect.

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